1. Field of the Invention
The present invention relates to a radiation image detector in which charges of a radiation image are generated and stored by receiving radiation carrying the radiation image, and the charges stored in the detector are detected as image signals.
2. Description of the Related Art
Various types of radiation image detectors are proposed and put into practical use in the medical and other industrial fields. In such a detector, a radiation image of a subject is recorded by receiving radiation transmitted through the subject, and the image recorded on the detector is detected by reading out the image signals in accordance with the image recorded on the detector.
Some of the radiation image detectors use, for example, a semiconductor material that generates charges when exposed to radiation, and some of them use a so-called TFT readout system.
One such TFT readout system radiation image detector includes, for example, a radiation image recording medium, which is a layer composite of a charge generating layer for generating charges by receiving radiation, and a charge detecting layer for storing the charges generated in the charge generating layer; and a detecting section having charge amplifiers for detecting charge signals flowed out from the radiation image recording medium and the like.
More specifically, the charge detecting layer of the radiation image recording medium includes multitudes of charge detecting elements disposed two-dimensionally in orthogonal directions, each having a storage section for storing the charge generated in the charge generating layer and a TFT switching element. The charge detecting layer further includes multitudes of charge signal lines, each being installed in parallel with each column of the charge detecting elements; and multitudes of gate control signal lines, each being installed in parallel with each row of the charge detecting elements and orthogonally to each of the charge signal lines.
When recording a radiation image using the radiation image detector constructed in the manner as described above, the radiation image is recorded by irradiating radiation carrying the image on the charge generating layer, and storing the charges generated in the charge generating layer in the storage sections of the charge detecting layer. When reading out the radiation image, a gate control signal is outputted to the gate control signal lines selectively from a gate driver, and the TFT switching elements of the charge detecting elements connected to the gate control signal line are switched to ON according to the gate control signal, and charge signals start to flow out to the charge signal lines from the storage sections of the activated charge detecting elements. The charge signals flowed out to the charge signal lines are detected as image signals through charge amplifiers and the like. In this way, the radiation image is read out.
Here, in the radiation image detector described above, the gate control signal lines and charge signal lines are disposed orthogonally to each other with an insulation layer between them. Consequently, a parasitic capacitor is formed between each of the gate control signal lines and each of the charge signal lines in the vicinity of the intersection. When the gate control signal flows through one of the gate control signal lines in the reading process for reading out the radiation image as described above, a potential difference is developed between the gate control signal line and each of the charge signal lines, and charges are stored in the parasitic capacitors. Then the charges stored in the parasitic capacitors flow out to the charge signal lines as noise signals and included in the charge signals flowed out from the storage sections of the charge detecting elements.
Under the circumstances described above, one method for eliminating the noise signals is proposed as described, for example, in U.S. patent application Publication No. 20040056204. In the method, noise compensation signal lines are provided orthogonally to the charge signal lines with an insulation layer between them, in addition to the gate control signal lines. Further, TFT switching elements, each connected to each of the noise compensation signal lines and each of the charge signal lines, and dummy capacitors, each connected to each of the TFT switching elements are also provided. When outputting the gate control signal to one of the gate control signal lines, a signal having an opposite polarity to that of the gate control signal is also outputted to the corresponding noise compensation signal line to generate a noise compensation signal in the vicinity of each intersection between each of the charge signal lines and the noise compensation signal line in accordance with the reverse polarity signal, and store it in each of the dummy capacitors, then the noise compensation signals stored in the dummy capacitors are outputted to the charge signal lines through the TFT switching elements to eliminate the noise signals.
However, each of the noise compensation signal lines described in U.S. patent application Publication No. 20040056204 is installed in a place which is different from the place where each of the gate control signal lines is installed. The insulation layer installed between the gate control signal lines and charge signal lines, and that installed between the noise compensation signal lines and charge signal lines have different thickness variations with each other. Consequently, each parasitic capacitor formed in the vicinity of the intersection between each of the gate control signal lines and each of the charge signal lines, and that formed in the vicinity of the intersection between each of the noise compensation signal lines and each of the charge signal lines may differ in the capacitance value. Accordingly, the noise signals and noise compensation signals may differ in magnitude and the noise may not be eliminated properly, resulting in the residual noise being included in the charge signals. Further, it requires a separate gate driver for outputting the reverse polarity signal to the noise compensation signal lines, causing the circuit to be more complicated and resulting in higher costs.